Semiconductor device and semiconductor assembly with lead-free solder

ABSTRACT

A semiconductor device includes a bump structure over a pad region. The bump structure includes a copper layer and a lead-free solder layer over the copper layer. The lead-free solder layer is a SnAg layer, and the Ag content in the SnAg layer is less than 1.6 weight percent.

TECHNICAL FIELD

This disclosure relates to lead-free solder, and more particularly, to a semiconductor device and a semiconductor assembly using the lead-free solder.

BACKGROUND

Modern integrated circuits are made up of literally millions of active devices such as transistors and capacitors. These devices are initially isolated from each other, but are later interconnected together to form functional circuits. Typical interconnect structures include lateral interconnections, such as metal lines (wirings), and vertical interconnections, such as vias and contacts. Interconnections are increasingly determining the limits of performance and the density of modern integrated circuits. On top of the interconnect structures, bond pads are formed and exposed on the surface of the respective chip. Electrical connections are made through bond pads to connect the chip to a package substrate or another die. Bond pads can be used for wire bonding or flip-chip bonding. Wafer level chip scale packaging (WLCSP) is currently widely used for its low cost and relatively simple processes. In a typical WLCSP, interconnect structures are formed on metallization layers, followed by the formation of under-bump metallurgy (UBM), and the mounting of solder balls.

Flip-chip packaging utilizes bumps to establish electrical contact between a chip's I/O pads and the substrate or lead frame of the package. Structurally, a bump actually contains the bump itself and a so-called under bump metallurgy (UBM) located between the bump and an I/O pad. An UBM generally contains an adhesion layer, a barrier layer and a wetting layer, arranged in this order on the I/O pad. The bumps themselves, based on the material used, are classified as solder bumps, gold bumps, copper pillar bumps and bumps with mixed metals. Recently, copper interconnect post technology is proposed. Instead of using solder bump, the electronic component is connected to a substrate by means of copper post. The copper interconnect post technology achieves finer pitch with minimum probability of bump bridging, reduces the capacitance load for the circuits and allows the electronic component to perform at higher frequencies. A solder alloy is still necessary for capping the bump structure and jointing electronic components as well.

Usually, a material used for the solder alloy is so-called Sn—Pb eutectic solder of Sn-38 mass % Pb. In recent years, it is urged to put Pb-free solder to practical use. The binary Sn—Ag alloy is used as the lead-free solder with Ag between 2.0˜4.5 weight percent, which melts at a temperature between 240˜260° C. The reflow soldering process and equipment for lead-free components are similar to conventional eutectic solder. Many developments of solder alloys have been directed to make the composition of an alloy, as close as possible to the eutectic composition of that in order to use the eutectic point to avoid thermal damage. However, the melting point of a Pb-free solder material is higher than that of the conventional Sn—Pb eutectic solder, so that there arises problems of cracks and stress reliability issues as TCB testing, especially to large die size. Even applied to the Cu post technology, the flip-chip assembly using the Pb-free solder material as the cap still suffers the crack issue induced by die edge/substrate interface stress.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects, features and advantages of this disclosure will become apparent by referring to the following detailed description of the preferred embodiments with reference to the accompanying drawings, wherein:

FIG. 1 is a cross-sectional diagram depicting an exemplary embodiment of a semiconductor device with a lead-free solder on a copper post structure;

FIG. 2 is a diagram showing the relationship between the Ag content in lead-free solder and bump hardness;

FIG. 3A and FIG. 3B are cross-sectional diagrams depicting an exemplary embodiment of a package assembly with the lead-free solder;

FIG. 4 is a cross-sectional diagram depicting an exemplary embodiment of a Cu post structure on Cu PPI for WLCSP application;

FIG. 5 is a cross-sectional diagram depicting an exemplary embodiment of a solder bump structure;

FIG. 6 is a cross-sectional diagram depicting an exemplary embodiment of a package assembly with the solder bump structure;

FIG. 7 is a cross-sectional diagram depicting an exemplary embodiment of a solder bump structure on Cu PPI for WLCSP application; and

FIG. 8A to FIG. 8C are cross-sectional diagrams depicting an exemplary embodiment of manufacturing vertically stacked devices using the lead-free solder.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

This disclosure provides a lead-free solder with controlled Ag content and reflow temperature used in semiconductor devices having Cu post, post passivation interconnects, solder bump, and/or through-silicon vias (TSVs) fabricated thereon, applied to flip-chip assembly, wafer-level chip scale package (WLCSP), three-dimensional integrated circuit (3D-IC) stack, and/or any advanced package technology fields. In the following description, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, one having an ordinary skill in the art will recognize that the disclosure can be practiced without these specific details. In some instances, well-known structures and processes have not been described in detail to avoid unnecessarily obscuring the disclosure. Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. It should be appreciated that the following figures are not drawn to scale; rather, these figures are merely intended for illustration.

Herein, cross-sectional diagram of FIG. 1 depicts an exemplary embodiment of a semiconductor device with a lead-free solder on a copper post structure.

An example of a substrate 10 used for Cu post interconnection fabrication may comprise a semiconductor substrate as employed in a semiconductor integrated circuit fabrication, and integrated circuits may be formed therein and/or thereupon. The semiconductor substrate is defined to mean any construction comprising semiconductor materials, including, but is not limited to, bulk silicon, a semiconductor wafer, a silicon-on-insulator (SOI) substrate, or a silicon germanium substrate. Other semiconductor materials including group III, group IV, and group V elements may also be used. The integrated circuits as used herein refer to electronic circuits having multiple individual circuit elements, such as transistors, diodes, resistors, capacitors, inductors, and other active and passive semiconductor devices. The substrate 10 further includes inter-layer dielectric layers and a metallization structure overlying the integrated circuits. The inter-layer dielectric layers in the metallization structure include low-k dielectric materials, un-doped silicate glass (USG), silicon nitride, silicon oxynitride, or other commonly used materials. The dielectric constants (k value) of the low-k dielectric materials may be less than about 3.9, or less than about 2.8. Metal lines in the metallization structure may be formed of copper or copper alloys. One skilled in the art will realize the formation details of the metallization layers.

A conductive region 12 is a top metallization layer formed in a top-level inter-layer dielectric layer, which is a portion of conductive routs and has an exposed surface treated by a planarization process, such as chemical mechanical polishing (CMP), if necessary. Suitable materials for the conductive region 12 may include, but are not limited to, for example copper, aluminum, copper alloy, or other mobile conductive materials. In one embodiment, the conductive region 12 is a pad region 12, which may be used in the bonding process to connect the integrated circuits in the respective chip to external features.

A passivation layer 14 is formed and patterned on the substrate 10 to partially cover the pad region 12. The passivation layer 14 has an opening 15 exposing a portion of the pad region 12. The passivation layer 14 may be formed of a non-organic material selected from un-doped silicate glass (USG), silicon nitride, silicon oxynitride, silicon oxide, and combinations thereof. Alternatively, the passivation layer 14 may be formed of a polymer layer, such as an epoxy, polyimide, benzocyclobutene (BCB), polybenzoxazole (PBO), and the like, although other relatively soft, often organic, dielectric materials may also be used.

An under-bump-metallurgy (UBM) layer 16 including a diffusion barrier layer 16 a and a seed layer 16 b is formed on a portion of the passivation layer 14 and electrically connected to the pad region 12 through the opening 15. As depicted, the UBM layer 28 directly contacts the exposed portion of the pad region 12 and lines the sidewalls and bottom of the opening 15. The diffusion barrier layer 16 a, also referred to as a glue layer, is formed to cover the sidewalls and the bottom of the opening 15. The diffusion barrier layer 16 a may be formed of tantalum nitride, although it may also be formed of other materials such as titanium nitride, tantalum, titanium, or the like. The formation methods include physical vapor deposition (PVD) or sputtering. The seed layer 16 b may be a copper seed layer formed on the diffusion barrier layer 16 a. The seed layer 16 b may be formed of copper alloys that include silver, chromium, nickel, tin, gold, and combinations thereof. In one embodiment, the UBM layer 16 is a Cu/Ti layer.

A copper (Cu) post 18 is formed on the UBM layer 16. As used throughout this disclosure, the term “copper (Cu) post” is intended to include substantially a post including pure elemental copper, copper containing unavoidable impurities, and copper alloys containing minor amounts of elements such as tantalum, indium, tin, zinc, manganese, chromium, titanium, germanium, strontium, platinum, magnesium, aluminum or zirconium. The formation methods may include sputtering, printing, electro plating, electroless plating, and commonly used chemical vapor deposition (CVD) methods. For example, electro-chemical plating (ECP) is carried out to form the Cu post with a thickness greater than 40 um. In other embodiments, the thickness of the Cu post is about 40˜70 μm, although the thickness may be greater or smaller.

A cap layer 20 is formed on the top surface of the Cu post 18. The cap layer 20 could act as a barrier layer to prevent copper in the Cu post 18 to diffuse into bonding material, such as solder alloy, that is used to bond the substrate 10 to external features. The prevention of copper diffusion increases the reliability and bonding strength of the package. The cap layer 20 may include nickel, tin, tin-lead (SnPb), gold (Au), silver, palladium (Pd), In, nickel-palladium-gold (NiPdAu), nickel-gold (NiAu), other similar materials, or alloy. In one embodiment, the cap layer 20 is a nickel layer with a thickness about 1˜5 um.

A lead-free (Pb-free) solder layer 22 is formed on the cap layer 20. Thus the lead-free solder layer 22, the cap layer 20, and the Co post 18 are referred to as a bump structure 25 formed over the pad region 12. The lead-free (Pb-free) solder layer 22 may be formed by plating and reflowing processes. In one embodiment, the lead-free solder layer 22 is formed as solder ball on the cap layer 20. In other embodiment, the lead-free solder layer 22 is a plated solder layer on the cap layer 20. For a lead-free solder system, the solder layer 22 is SnAg with Ag content being controlled lower than 1.6 weight percent (wt %). In the reflow process, the melting temperature of the lead-free solder layer 22 is accordingly adjusted to the range of between about 240° C. to about 280° C. In one embodiment, the Ag content in the lead-free solder layer 22 is at the range between about 1.2 wt % to about 1.6 wt %. In other embodiment, the Ag content in the lead-free solder layer 22 is about 1.5 wt %. Reliability of package using lead-free solder alloy relates to several factors, including bump hardness and formation of intermetallic compounds (IMCs) and voids, which may potentially contribute to crack formation and cause thermo-mechanical stresses on the solder joint. It is observed that the bump becomes softer as the Ag content in the lead-free solder is lower as schematically shown in FIG. 2. The softer bump may eliminate the crack issue caused by thermal stress. The SEM study shows that the lead-free solder with lower Ag content provides better performance for crack resistance. On the contrary, as the Ag content in the lead-free solder is higher, the formation of IMCs, voids, and induced cracks will be observed at time zero. However, if the Ag content of the lead-free solder is lower than 1.2 wt %, only a part of the solder bump transfers the state from melting to solid when the temperature is lower than the melting point during the cooling step of reflow process, which makes thermal stress gathering at those bumps to cause cracks. As to the reflowing temperature of the lead-free solder layer 22 in the reflow process, it is adjusted at the range between about 240° C. to about 280° C. to avoid an uncompleted ball-shape formation and suppress the voids and IMCs formation.

FIG. 3A and FIG. 3B are cross-sectional diagrams depicting an exemplary embodiment of a package assembly with the lead-free solder.

The substrate 10 may then be sawed and packaged onto a package substrate, or another die, with solder balls or Cu posts mounted on a pad on the package substrate or the other die. The structure shown in FIG. 1 is flipped upside down and attached to another substrate 100 at the bottom. The substrate 100 may be a package substrate, board (e.g., a print circuit board (PCB)), or other suitable substrate. The connection structure 102 contacts the substrate 100 at various conductive attachment points, for example, a pre-solder layer 106 on contact pads 104 and/or conductive traces. The pre-solder layer 106 may be a eutectic solder material including alloys of tin, lead, silver, copper, nickel, bismuth, or combinations thereof. Using an exemplary coupling process including a flux application, chip placement, reflowing of melting solder joints, and cleaning of flux residue, a joint-solder structure 108 is formed between the substrates 10 and 100. After the assembly process, the joint-solder layer 108 includes the lead-free solder mixed with the pre-solder, in which the Ag content in the joint-solder layer 108 is lower than 3.0 wt %. The substrate 10, the joint-solder layer 108, and the other substrate 100 may be referred to as a packaging assembly, or in the present embodiment, a flip-chip assembly.

FIG. 4 is a cross-sectional diagram depicting an exemplary embodiment of a Cu post structure on Cu PPI for WLCSP application, while explanation of the same or similar portions to the description in FIG. 1 to FIG. 3 will be omitted.

Compared with the Cu post 18 and the UBM layer 16 formed over the pad region 12 as depicted in FIG. 1, the WLCSP process forms a post passivation interconnect (PPI) line 26 underlying the UBM layer 16 and the passivation layer 14 and overlying another passivation layer 24. One end of the PPI line 26 is electrically connected to the pad region 12 (not shown in FIG. 4), and the other end of the PPI line 26 is electrically connected to the UBM layer 16 and the Cu post 18. In one embodiment, one portion of the PPI line 26 is exposed by the passivation layer 14, on which the UBM layer 16 is directly formed. The passivation layer 24 may be formed of a non-organic material selected from un-doped silicate glass (USG), silicon nitride, silicon oxynitride, silicon oxide, and combinations thereof. Alternatively, the passivation layer 24 may be formed of a polymer layer, such as an epoxy, polyimide, benzocyclobutene (BCB), polybenzoxazole (PBO), and the like, although other relatively soft, often organic, dielectric materials may also be used. The PPI line 26 may include, but not limited to, for example copper, aluminum, copper alloy, or other mobile conductive materials. The PPI line 26 may further include a nickel-containing layer (not shown) on the top a copper-containing layer. The PPI formation methods include plating, electroless plating, sputtering, chemical vapor deposition methods, and the like. The PPI line 26 may also function as power lines, re-distribution lines (RDL), inductors, capacitors or any passive components. The PPI line 26 may have a thickness less than about 30 μm, for example between about 2 μm and about 25 μm.

FIG. 5 is a cross-sectional diagram depicting an exemplary embodiment of a solder bump structure, while explanation of the same or similar portions to the description in FIG. 1 to FIG. 1 will be omitted.

Compared with the structure as depicted in FIG. 1, the formation of the Cu post 18 is replaced by a thin copper (Cu) layer 18 a in the solder bump process followed by the formation of the cap layer 20 a and the lead-free solder layer 22 a. The thin Cu layer 18 a has a thickness relatively thinner than the Cu post 18. The thin Cu layer 18 a has a thickness less than 10 um. In an embodiment, the thin Cu layer 18 a has a thickness about 1˜10 um, for example about 4˜6 um, although the thickness may be greater or smaller. The thin Cu layer formation methods may include sputtering, printing, electro plating, electroless plating, and commonly used chemical vapor deposition (CVD) methods. The lead-free (Pb-free) solder layer 22 a is reflowed as a solder ball. Therefore, the lead-free solder layer 22 a, the cap layer 20 a and the copper layer 18 a are referred to as a solder bump structure 25 a. The solder layer 22 a is SnAg with Ag content being controlled lower than 1.6 weight percent (wt %), which is melted at the range of between about 240° C. to about 280° C. In one embodiment, the Ag content in the lead-free solder layer 22 a is at the range between about 1.2 wt % to about 1.6 wt %. In other embodiment, the Ag content in the lead-free solder layer 22 a is about 1.5 wt %.

FIG. 6 is a cross-sectional diagram depicting an exemplary embodiment of a package assembly with the semiconductor device shown in FIG. 5, while explanation of the same or similar portions to the description in FIG. 1 to FIG. 3 will be omitted. After the assembly process, the substrate 10, the joint-solder layer 108, and the other substrate 100 may be referred to as a packaging assembly, or in the present embodiment, a flip-chip assembly. The joint-solder layer 108 includes the lead-free solder mixed with the pre-solder, in which the Ag content in the joint-solder layer 108 is lower than 3.0 wt %.

FIG. 7 is a cross-sectional diagram depicting an exemplary embodiment of a solder bump structure on Cu PPI for WLCSP application, while explanation of the same or similar portions to the description in FIG. 1 to FIG. 6 will be omitted. Compared with the solder bump structure as depicted in FIG. 5, the WLCSP process forms a post passivation interconnect (PPI) line 26 underlying the UBM layer 16 and the passivation layer 14 and overlying another passivation layer 24. One end of the PPI line 26 is electrically connected to the pad region 12 (not shown in FIG. 7), and the other end of the PPI line 26 is electrically connected to the UBM layer 16. In one embodiment, one portion of the PPI line 26 is exposed by the passivation layer 14, on which the UBM layer 16 is directly formed.

FIG. 8A to FIG. 8C are cross-sectional diagrams depicting an exemplary embodiment of manufacturing vertically stacked devices using the lead-free solder. Three-dimensional (3D) wafer-to-wafer, die-to-wafer or die-to-die vertical stack technology seeks to achieve the long-awaited goal of vertically stacking many layers of active IC devices such as processors, programmable devices and memory devices to shorten average wire lengths, thereby reducing interconnect RC delay and increasing system performance. One major challenge of 3D interconnects on a single wafer or in a die-to-wafer vertical stack is through-silicon via (TSV) that provides a signal path for high impedance signals to traverse from one side of the wafer to the other. Through-silicon via (TSV) is typically fabricated to provide the through-silicon via filled with a conducting material that passes completely through the layer to contact and connect with the other TSVs and conductors of the bonded layers.

Referring to FIG. 8A, a wafer 200, which includes a substrate 210, is provided. An example of a substrate 210 may include a semiconductor substrate as employed in a semiconductor integrated circuit fabrication, and integrated circuits may be formed therein and/or thereupon. The substrate 210 has a first surface 210 a and a second surface 210 b opposite to the first surface 210 a. The first surface 210 a may be referred to as the frontside on which integrated circuits including active and passive devices such as transistors, resistors, capacitors, diodes, inductors and the like, are formed to connect bond pads and/or other interconnection structures. The circuitry may be any type of circuitry suitable for a particular application. The functions may include memory structures, processing structures, sensors, amplifiers, power distribution, input/output circuitry, or the like. The second surface 210 b may be referred to as the backside, which will be thinned down and processed to form bond pads and/or other interconnection structures thereon.

A first dielectric layer 214 is formed on the first surface 10 a, in which contacts are formed to electrically connect the devices respectively. Generally, the first dielectric layer 214 may be formed, for example, of a low-K dielectric material, silicon oxide, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), fluorinated silicate glass (FSG), or the like, by any suitable method known in the art. Other materials and processes may be used.

A plurality of through vias 216 passes through at least a part of the substrate 210. The through via 216 is a conductor-filled plug extending from the first surface 210 a toward the second surface 10 b and reaching an intended depth. Also, an isolation layer is formed on the sidewalls and bottom of the though via 216, and insulates the though via 216 from the substrate 210. The through vias 16 may be formed of any suitable conductive material, but are preferably formed of a highly-conductive, low-resistive metal, elemental metal, transition metal, or the like. In an embodiment, the though via 216 is a trench filled with a conductive layer formed of Cu, W, Cu alloy, or the like. A conductive barrier layer formed of Ti, TiN, Ta, TaN or combinations thereof may be formed in the trench surrounding the conductive layer. The isolation layer may be formed of commonly used dielectric materials such as silicon nitride, silicon oxide (for example, tetra-ethyl-ortho-silicate (TEOS) oxide), and the like.

A first interconnect structure 218, which includes inter-layer dielectric layers and a metallization structure are formed overlying the integrated circuits, the first dielectric layer 214 and the through vias 216. The inter-layer dielectric layers in the metallization structure include low-k dielectric materials, un-doped silicate glass (USG), silicon nitride, silicon oxynitride, or other commonly used materials. The dielectric constants (k value) of the low-k dielectric materials may be less than about 3.9, or less than about 2.8. The metallization structure includes metal lines and vias, which may be formed of copper or copper alloys, and may be formed using the well-known damascene processes. One skilled in the art will realize the formation details of the metallization layers.

A passivation layer 220 is formed over first interconnect structure 218. The passivation layer 220 may be formed of materials such as silicon oxide, silicon nitride, un-doped silicate glass (USG), polyimide, and/or multi-layers thereof. A metal pad 222 is formed on the passivation layer 220. The metal pad 222 may be formed of aluminum, copper, silver, gold, nickel, tungsten, alloys thereof, and/or multi-layers thereof. The metal pad may be electrically connected to the devices and the through via 216, for example, through underlying first interconnection structure 218. A dielectric buffer layer 224 is formed on the metal pad and patterned to provide a bump formation window. The dielectric buffer layer 224 may be formed of a polymer, such as an epoxy, polyimide, benzocyclobutene (BCB), polybenzoxazole (PBO), and the like, although other relatively soft, often organic, dielectric materials can also be used. A bump structure 226 is then formed overlying and electrically connected to the metal pad 222. The bump structure 224 refers to the structures 25 and 25 a shown in FIGS. 1, 4,5 and 7. The details are thus not repeated herein.

Referring to FIG. 8B, the wafer 200 is attached to a carrier 300 through an adhesive layer 302 and then the bonded structure is flipped. Since the bonded structure is flipped, a thinning process e.g., grinding and/or etching is then performed on the second surface 210 b to remove the most portion of the substrate 210 to the desired final thickness, resulting in a thinned substrate 210″ with a predetermined thickness depending on the purpose for which the semiconductor package is used. One of ordinary skill in the art will realize that other thinning processes, such as a polish process (including a wet polish (CMP) and a dry polish), a plasma etch process, a wet etch process, or the like, may also be used. In an embodiment, the end 216 a of the through via 216 is exposed and/or protruded from the second surface 210 b″ of the thinned substrate 210″ after the thinning process. Then, processing of the second surface 210 b″ of the thinned substrate 210″ is performed to form a second interconnect structure 228 electrically connected to the through via 216. For an example, the second interconnect structure 228 includes electrical connections and/or other structures (e.g., redistribution layers, pads, solder bumps or copper bumps) are formed over the surface 210 b″ of the thinned wafer 210″. The details of the backside grinding and the interconnect structure formation is provided in the co-pending U.S. patent applications: application Ser. No. 12/332,934, entitled “Backside Connection to TSVs Having Redistribution Lines;” and application Ser. No. 12/347,742, entitled “Bond Pad Connection to Redistribution Lines Having Tapered Profiles;” which applications are hereby incorporated herein by reference. The details are thus not repeated herein.

Next, dies 400 are provided for being bonded onto the thinned wafer 210″. The dies 400 may be memory chips, RF (radio frequency) chips, logic chips, or other chips. Each die 400 includes a bump structure 402 used for electrically connected to the second interconnect structure 228 of the thinned wafer 210″. The bump structure 402 includes a copper layer 404, an optional cap layer 406 on the top of the copper layer 404, and a lead-free solder layer 408 over the copper layer 404. The copper layer 404 may be a thin copper layer of about 0.5˜10 um thickness or a thick copper layer of about 40˜70 μm thickness. The optional cap layer 406 may include nickel, gold (Au), silver, palladium (Pd), indium (In), nickel-palladium-gold (NiPdAu), nickel-gold (NiAu) or other similar materials or alloy. The lead-free solder layer 408 may be a plated layer or reflowed as a solder ball. The solder layer 408 is SnAg with Ag content being controlled lower than 1.6 weight percent (wt %). In the reflow process, the melting temperature of the lead-free solder layer 408 is accordingly adjusted to the range of between about 240° C. to about 280° C. In one embodiment, the Ag content in the lead-free solder layer 408 is at the range between about 1.2 wt % to about 1.6 wt %. In other embodiment, the Ag content in the lead-free solder layer 408 is about 1.5 wt %.

Referring to FIG. 8C, the dies 400 are bonded onto the thinned wafer 210″ through the bump structures 402 and the second interconnect structure 228, forming a dies-to-wafer stack 500. Using an exemplary coupling process including a flux application, chip placement, reflowing of melting solder joints, and cleaning of flux residue, joint structures 502 are formed between the wafer 210″ and the dies 400. The joint structure 502 includes the bump structure 402, the second interconnect structure 228, and the lead-free solder layer 408 joined therebetween. Thereafter, the carrier 300 is detached from the thinned wafer 210″, and then dies-to-wafer stack 500 is sawed in the usual manner along cutting lines to separate the dies-to-wafer stack into individual IC stacks, and then packaged on to a package substrate with solder bumps or Cu bumps mounted on a pad on the package substrate. In some embodiments, the package substrate is replaced by another die.

In the preceding detailed description, the disclosure is described with reference to specifically exemplary embodiments thereof. It will, however, be evident that various modifications, structures, processes, and changes may be made thereto without departing from the broader spirit and scope of the disclosure, as set forth in the claims. The specification and drawings are, accordingly, to be regarded as illustrative and not restrictive. It is understood that the disclosure is capable of using various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. 

1. A semiconductor device, comprising: a semiconductor substrate; a pad region on the semiconductor substrate; and a bump structure overlying and electrically connected to the pad region; wherein the bump structure comprises a copper layer and a SnAg layer overlying the copper layer, and the Ag content in the SnAg layer is less than 1.6 weight percent.
 2. The semiconductor device of claim 1, wherein the Ag content in the SnAg layer is greater than 1.2 weight percent.
 3. The semiconductor device of claim 1, wherein the Ag content in the SnAg layer is 1.5 weight percent.
 4. The semiconductor device of claim 1, wherein the copper layer is a copper post with a thickness greater than 40 um.
 5. The semiconductor device of claim 1, wherein the copper layer has a thickness less than 10 um.
 6. The semiconductor device of claim 1, further comprising a nickel layer between the copper layer and the SnAg layer.
 7. The semiconductor device of claim 1, further comprising an under-bump metallization (UBM) layer between the bump structure and the pad region.
 8. The semiconductor device of claim 1, further comprising a through via passing through the semiconductor substrate and electrically connected to the pad region.
 9. The semiconductor device of claim 1, wherein the through via comprises copper.
 10. The semiconductor device of claim 1, further comprising an interconnect line between the bump structure and the pad region.
 11. A semiconductor assembly comprising: a first substrate; a second substrate; and a joint structure disposed between the first substrate and the second substrate; wherein the joint structure comprises a bump structure between the first substrate and the second substrate and a solder layer between the bump structure and the second substrate; and wherein the solder layer comprises silver (Ag), and the Ag content in the solder layer is less than 3.0 weight percent.
 12. The semiconductor assembly of claim 11, wherein the bump structure comprises a copper post with a thickness greater than 40 um.
 13. The semiconductor assembly of claim 12, wherein the bump structure comprises a nickel-containing layer on the copper post.
 14. The semiconductor assembly of claim 11, wherein the bump structure comprises has a copper layer with a thickness less than 10 um.
 15. The semiconductor assembly of claim 14, wherein the bump structure comprises a nickel-containing layer on the copper layer.
 16. The semiconductor assembly of claim 11, wherein at least one of the first substrate and the second substrate is a semiconductor substrate comprising a through via passing through the semiconductor substrate and electrically connected to the bump structure.
 17. The semiconductor assembly of claim 11, wherein each of the first substrate and the second substrate is a semiconductor substrate.
 18. A method of forming a semiconductor device, comprising: providing a semiconductor substrate; forming a pad region overlying the semiconductor substrate; forming a copper post over the pad region; forming a lead-free solder layer over the copper post, wherein the lead-free solder layer comprises silver (Ag) and the Ag content in the lead-free solder layer is less than 1.6 weight percent; and reflowing the lead-free solder layer at the temperature between 240° C. to 280° C.
 19. The method of claim 18, wherein reflowing the lead-free solder layer is at the temperature between 240° C. to 280° C.
 20. The method of claim 18, wherein the Ag content in the lead-free solder layer is greater than 1.2 weight percent. 